Lubricant oil formulations generally contain viscosity index (“VI”) improving components derived from ethylene-alpha-olefin copolymers, which modify the rheological behavior to increase the lubricant viscosity, and promote a more constant viscosity over the range of temperatures over which the lubricant is used in, for example, automotive engines. Higher ethylene-content copolymers efficiently promote oil thickening, shear stability and low temperature viscometrics, while lower ethylene-content copolymers are added for the purpose of lowering the oil pour point. Typical viscosity index improvers can be functionalized or non-functionalized olefin polymerization products. With functionalization, polar groups are included to provide a dispersant effect for any debris in the oil, such as that generated by engine wear. In a typical process, the VI product is functionalized with maleic anhydride, and optionally is further derivatized by reaction with an amine compound.
Blends of amorphous and semicrystalline ethylene propylene copolymers are known for lubricant oil formulations. The combination of two such ethylene-propylene copolymers allows for increased thickening efficiency, shear stability, low temperature viscosity performance and pour point. See, e.g., U.S. Pat. No. 5,391,617, and EP 0 638,611, the disclosures of which are incorporated herein by reference for purposes of U.S. patent practice.
Using conventional vanadium based Ziegler-Natta catalysts, the general approach is to separately polymerize the two copolymers in an alkane solvent, either hexane in a solution process or propylene in a slurry process, and to finish the polymers to remove the solvent. The two copolymer components have a medium viscosity, and a molecular weight in excess of that needed in the final lubricant formulation, for two reasons. First, most of the traditional EPDM manufacturing plants cannot “finish” low viscosity polymers having the right viscosity for lubricant formulations; and second, low viscosity copolymers tend to cold flow upon storage. The second issue is particularly true for the amorphous copolymers, which have a lower plateau modulus. The bales are then processed by a series of steps to create the final lubricant composition.
In the most generally practiced procedure, the amorphous and semicrystalline components are dissolved in oil at about a 10 wt % concentration, and masticated in the presence of oxygen to obtain overall a scissioning effect which reduces the initial molecular weight. This conventional procedure suffers from several disadvantages. Mastication is a batch process which can take many hours. In addition, the presence of oxygen generates a safety hazard. Moreover, the concentrate then may have to be shipped to the lubricant oil formulator, which generates a long supply line and significant transport costs and so mitigates against the use of blends of amorphous and semicrystalline ethylene propylene copolymers.
In another procedure, the VI improver supplier blends the copolymer components in an extruder, and in that process reduces the blend viscosity by shearing to reduce the initial molecular weight. In both procedures, the medium viscosity of the starting material (and more specifically the amorphous copolymer) is necessary so that the composition does not “cold flow” during storage.
It would be desirable, but has not been heretofore possible, to more directly produce a lubricant composition containing a VI improver in order to reduce overall costs of the totality of steps needed, reduce shipping costs, minimize safety hazards and reduce quality control problems. It would be desirable especially to arrive at the target molecular weight without having to pass through blending or masticating steps which have a molecular weight reducing effect, and to arrive at a composition having the correct viscosity and composition for VI improver applications, without the extra cost and complexity of blending and viscosity-reducing shearing steps. It would further be desirable to determine the copolymer composition to reflect the need for solubility and viscosity improvement. It would still further be desirable to have the capability of making in polymerization, a polymer of a molecular weight which is also desired for the final lubricant formulation, without having to use blending steps which may, intentionally but unavoidably, have a lowering effect on the molecular weight.
Thus, there is a need for processes to produce polymer compositions for VI improvers, and VI improver compositions, which overcome these and other disadvantages of prior art processes for making lubricant formulations and the compositions and ingredients used therein.